Reducing mercury emissions from the burning of coal

ABSTRACT

Sorbent components containing halogen, calcium, alumina, and silica are used in combination during coal combustion to produce environmental benefits. Sorbents such as calcium bromide are added to the coal ahead of combustion and other components are added into the flame or downstream of the flame, preferably at minimum temperatures to assure complete formation of the refractory structures that result in various advantages of the methods. When used together, the components
         reduce emissions of elemental and oxidized mercury;   increase the level of Hg, As, Pb, and/or Cl in the coal ash;   decrease the levels of leachable heavy metals (such as Hg) in the ash, preferably to levels below the detectable limits; and   make a highly cementitious ash product.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/530,364 filed on Jun. 22, 2012 now U.S. Pat. No. 8,501,128 issuedAug. 6, 2013; which is a continuation of U.S. application Ser. No.13/098,973 filed May 2, 2011 now U.S. Pat. No. 8,226,913 issued Jul. 24,2012; which is a continuation of U.S. application Ser. No. 12/813,214filed Jun. 10, 2010 now U.S. Pat. No. 7,955,577 issued Jun. 7, 2011;which is a continuation of U.S. application Ser. No. 11/886,269 filedSep. 13, 2007 now U.S. Pat. No. 7,758,827, which issued Jul. 20, 2010;which is the National Phase filing of PCT/US2006/010000 filed Mar. 16,2006; which claims the benefit of U.S. Provisional Application No.60/765,944 filed Feb. 7, 2006 and U.S. Provisional Application No.60/759,994 filed Jan. 18, 2006; and also claims the benefit of U.S.Provisional Application No. 60/742,154 filed Dec. 2, 2005 and U.S.Provisional Application No. 60/662,911 filed Mar. 17, 2005. The entiredisclosures of each of the above applications are hereby incorporated byreference.

INTRODUCTION

The invention provides compositions and methods for reducing the levelsof mercury emitted into the atmosphere upon burning ofmercury-containing fuels such as coal. In particular, the inventionprovides for addition of various halogen and other sorbent compositionsinto the coal burning system during combustion.

Significant coal resources exist around the world capable of meetinglarge portions of the world's energy needs into the next two centuries.High sulfur coal is plentiful, but requires remediation steps to preventexcess sulfur from being released into the atmosphere upon combustion.In the United States, low sulfur coal exists in the form of low BTUvalue coal in the Powder River basin of Wyoming and Montana, in lignitedeposits in the North Central region of North and South Dakota, and inlignite deposits in Texas. But even when coals contain low sulfur, theystill contain non-negligible levels of elemental and oxidized mercuryand/or other heavy metals.

Unfortunately, mercury is at least partially volatilized upon combustionof coal. As a result, the mercury tends not to stay with the ash, butrather becomes a component of the flue gases. If remediation is notundertaken, the mercury tends to escape from the coal burning facilityinto the surrounding atmosphere. Some mercury today is captured byutilities, for example in wet scrubber and SCR control systems. However,most mercury is not captured and is therefore released through theexhaust stack.

Mercury emissions into the atmosphere in the United States areapproximately 50 tons per year. A significant fraction of the releasecomes from emissions from coal burning facilities such as electricutilities. Mercury is a known environmental hazard and leads to healthproblems for both humans and non-human animal species. To safeguard thehealth of the public and to protect the environment, the utilityindustry is continuing to develop, test, and implement systems to reducethe level of mercury emissions from its plants. In combustion ofcarbonaceous materials, it is desirable to have a process whereinmercury and other undesirable compounds are captured and retained afterthe combustion phase so that they are not released into the atmosphere.

In addition to wet scrubber and SCR control systems that tend to removemercury partially from the flue gases of coal combustion, other methodsof control have included the use of activated carbon systems. Use ofsuch systems tends to be associated with high treatment costs andelevated capital costs. Further, the use of activated carbon systemsleads to carbon contamination of the fly ash collected in exhaust airtreatments such as the bag house and electrostatic precipitators.

At the same time, demand for cementitious materials such as portlandcement is expected to increase as developed countries maintain theirinfrastructure and developing countries build and maintain roads, dams,and other major constructions for the benefit of their citizens.

When coal is burned to produce heat energy from combustion ofcarbonaceous material, the unburned material and particulate combustionproducts form an ash with pozzolanic and/or cementitious properties.While the chemical composition of coal ash depends on the chemicalcomposition of the coal, the ash normally contains a major amount ofsilica and alumina, and significant but lesser amount of calcium.

So-called fly ash produced from burning of pulverized coal in a coalfired furnace or boiler is a powdery particulate matter made of thecomponents of coal that do not volatize upon combustion. The ash isnormally carried off in the flue gas and is usually collected from theflue gas using conventional apparatus such as electrostaticprecipitators, filtration devices such as bag houses, and/or mechanicaldevices such as cyclones. The burning of coal entails the production ofa large amount of coal ash, which must be dealt with by the coal burningfacility. For example, under certain circumstances ash from burning coalhas been successfully used in portland cement concrete as a partialreplacement for portland cement. Coal ash has further been used acomponent in the production of flowable fill and as a component asstabilized base and sub-based mixtures. In these applications, theamount of ash used, especially as a replacement for portland cement insuch applications, is limited by the cementitious nature or lack thereofof the particular ash product.

Even though reuse of the ash is preferred for economic reasons, in manysituations, the ash is not suitable to be used as a component ofcementitious mixtures. In many cases the ash must be land filled orotherwise treated as a waste product.

Methods and compositions for burning coal to produce an ash producthaving highly cementitious qualities would be a significant advance,because it would both reduce costs of waste disposal from coal burningutilities and reduce the cost of concrete products for needed buildingprojects.

SUMMARY

Processes and compositions are provided for decreasing emissions ofmercury upon combustion of fuels such as coal. Various sorbentcompositions are provided that contain components that reduce the levelof mercury and/or sulfur emitted into the atmosphere from suchcombustion. In various embodiments, use of the sorbent compositionsleads to a fly ash combustion product from which mercury or other heavymetals do not significantly leach under acidic conditions.

In various embodiments, the sorbent compositions are added directly tothe fuel before combustion; into the furnace or fireball while the fuelis burning; into the flue gas post combustion zone; or in variouscombinations. The sorbent compositions comprise a source of calcium,alumina, and silica, preferably in the form of alkaline powders. Invarious embodiments, use of a sorbent containing calcium, silica, andalumina as alkaline powders lowers the amount of sulfur and/or mercuryemitted from the facility into the atmosphere. In one aspect, use of thealkaline powders lowers the oxidized mercury, for example in systemswhere the flame temperature is low, for example down to about 1000° F.

In a preferred embodiment, the sorbent also includes a source ofhalogen, and/or a sorbent containing a source of halogen is separatelyadded into the coal-burning system. Among the halogens, iodine andbromine are preferred. In various embodiments, inorganic bromides makeup a part of the sorbent compositions. In various embodiments mercurysorbent compositions containing halogen, especially bromine and/oriodine, are added to the fuel as a powder or a liquid prior tocombustion. Alternatively, the sorbent compositions containing halogensuch as bromine and iodine are injected into the flue gas at a pointafter the combustion chamber where the temperature is higher than about500° C. (932° F.), preferably higher than 1500° F. (about 800° C.),and/or into the furnace during combustion.

In preferred embodiments, mercury emissions from coal burning facilitiesare reduced to such an extent that 90% or more of the mercury in thecoal is captured before release into the atmosphere. Most of the mercuryis captured in the fly ash in non-leaching form; corrosion from sulfurgases is also reduced. In preferred embodiments, significant sulfurreduction is achieved.

Methods and compositions are provided for burning coal to produce an ashproduct that is highly cementitious in nature. In various embodiments,the cementitious nature of the ash allows for formulation of portlandcement concrete and similar products with up to 50% or more of theportland cement being substituted by the ash product. In variousembodiments, the strength activity index of portland type cementproducts formulated with up to 50% or more ash is greater than 75% andpreferably 100% or greater. Accordingly, in some embodiments, the ashproduct of the invention is used as the main cementitious material inportland cement concretes, in stabilized base, in sub-base mixtures, inflowable fill (also called controlled low strength material or CLSM),and the like.

The fly ash produced from combusting coal with these sorbent componentsis generally higher in calcium content than the specifications for classF or class C fly ash, and the combined content of silica, alumina, andiron oxide, while significant, is considerably below the specificationsfor class F and class C fly ash.

In various embodiments, the invention provides a variety of cementproducts such as portland cement concrete, flowable fill, stabilizedbase, and similar products in which the conventional cement (portlandcement) otherwise used in the products is replaced in whole or in partwith the cementitious ash product described herein. In particular, inpreferred embodiments, the cementitious ash product of the currentdisclosure is used to replace 40% or more of the portland cementconventionally used in such products.

In various embodiments, use of the cementitious ash in building productsas total or partial replacement for portland cement results in reducedcarbon dioxide emissions that would otherwise result from themanufacture of portland cement. In addition to avoided carbon dioxideemissions from calcining of limestone to make portland cement and theburning of fossil fuels to provide the energy needed to make portlandcement, use of the sorbent components tends to increase the efficiencyof energy production from burning of coal, further reducing greenhouseemissions from the burning of fossil fuel to produce energy.

DESCRIPTION

Sorbents, sorbent components and methods for their use are describedherein; in U.S. Provisional Application No. 60/662,911 filed Mar. 17,2005; in U.S. Provisional Application No. 60/742,154 filed Dec. 2, 2005;in U.S. Provisional Application No. 60/759,994 filed Jan. 18, 2005; andin U.S. Provisional Application No. 60/765,944 filed Feb. 7, 2006, thedisclosures of which are hereby incorporated by reference. Apparatus andmethods of injection of the various sorbent compositions are describedherein and in U.S. Provisional Applications No. 60/759,943 filed Jan.18, 2006 and No. 60/760,424 filed Jan. 19, 2006, the disclosures ofwhich are incorporated by reference.

In various embodiments, the invention provides compositions and methodsfor reducing emissions of mercury that arise from the combustion ofmercury-containing fuels such as coal. A commercially valuableembodiment is use of the invention to reduce sulfur and/or mercuryemissions from coal burning facilities to protect the environment andcomply with government regulations and treaty obligations.

In various embodiments, the methods prevent release of mercury into theatmosphere from point sources, such as coal-burning utilities bycapturing the mercury in the ash. Further, the methods prevent releaseof mercury and other heavy metals into the environment by leaching fromsolid wastes such as coal ash produced by burning the mercury containingcoal. In both these ways, mercury is kept out of bodies of water. Thus,prevention or reduction of mercury emissions from such facilities ascoal-burning utilities leads to a variety of environmental benefits,including less air pollution, less water pollution, and less hazardouswaste production, with less resulting ground contamination. Forconvenience but without limitation, advantageous features of theinvention are illustrated as preventing air, water, and ground pollutionby mercury or other heavy metals.

Various sorbent components are used in combination to treat coal aheadof combustion and/or to be added into the flame or downstream of theflame, preferably at minimum temperatures to assure complete formationof the refractory structures that result in various advantages of themethods. The sorbent components comprise calcium, alumina, silica, andhalogen. In various embodiments, together, the components

-   -   reduce emissions of mercury and sulfur;    -   reduce emissions of elemental and oxidized mercury;    -   increase the efficiency of the coal burning process through        de-slagging of boiler tubes;    -   increase the level of Hg, As, Pb, and/or Cl in the coal ash;    -   decrease the levels of leachable heavy metals (such as Hg) in        the ash, preferably to levels below the detectable limits; and    -   make a highly cementitious ash product.

By calcium is meant a compound or composition that has a non-negligibleamount of calcium. For example, many alkaline powders contain 20% ormore calcium, based on CaO. Examples are limestone, lime, calcium oxide,calcium hydroxide (slaked lime), portland cement and other manufacturedproducts or by-products of industrial processes, and calcium-containingaluminosilicate minerals. Silica and alumina content is based on SiO₂and Al₂O₃ equivalents, even though it is appreciated that silica andalumina are often present in a more complex chemical or molecular form.

As used herein, all percentages are on a weight basis, unless indicatedas otherwise. It should be noted that the chemical compositions ofvarious materials described herein are expressed in terms of simpleoxides calculated from elemental analysis, typically determined by x-rayfluorescence techniques. While the various simple oxides may be, andoften are, present in more complex compounds in the material, the oxideanalysis is a useful method for expressing the concentration ofcompounds of interest in the respective compositions.

Although much of the following discussion will refer to coal as thefuel, it is to be understood that the description of coal burning is forillustrative purposes only and the invention is not necessarily to belimited thereby. For example, other types of facilities that burn fuelswith potentially harmful levels of mercury or other heavy metals includeincineration plants, such as those used to incinerate household waste,hazardous waste, or sewage sludge. In addition, many facilities burnfuel mixtures that comprise coal as well as other fuels, such as naturalgas, synthetic gas, or waste-derived fuels.

A variety of waste streams are incinerated in such plants, which oftenoperates in populated areas for logistical reasons. Household waste cancontain mercury from a variety of sources, such as discarded batteriesand thermometers as well as a wide variety of consumer items withdetectable mercury levels. Hazardous waste streams include mercury froma number of commercial or industrial sources. Sewage sludge containsmercury resulting from ingestion and elimination of mercury-containingfoods and from other sources. All of the waste streams also containmercury from a number of natural sources as well. When burned in anincinerator, the wastes can release volatile mercury or mercurycompounds into the air, which tend to settle to the ground close to theincineration plant, leading to local contamination of the soil andgroundwater, as well as lowered air quality. Accordingly, in variousembodiments of the invention, waste streams containing mercury or otherheavy metals are incinerated in the presence of various mercury sorbentsadded into the incineration system as described below. In preferredembodiments, halogen and preferably silica and alumina are added insufficient amounts to reduce mercury emissions into the atmosphere andto render mercury non-leachable that is captured in the ash.

Major elements in coal, besides carbon, include silica, alumina, andcalcium, along with lesser amounts of iron. In addition, trace heavymetals such as arsenic, antimony, lead, chromium, cadmium, nickel,vanadium, molybdenum, manganese, copper, and barium are normallypresent. These elements tend to report to the ash upon combustion ofcoal. Coal also contains significant amounts of sulfur. Upon combustion,the sulfur in coal burns to produce volatile sulfur oxides, which tendto escape from the coal burning utility in gaseous form. It is desiredto remediate or reduce the level of sulfur oxides emitted from coalburning plants.

Coal also contains mercury. Although present at a low level, mercurytends to volatilize during combustion and escape from the coal burningutility. Even at the low levels produced from the combustion of coal,the release of mercury into the environment is undesirable because theelement is toxic and tends to accumulate in body tissues. Because ofmercury's damaging effect on health and the environment, its release hasrecently come under regulatory control in the United States andelsewhere in the world. Whether mercury is subject to regulatorycontrols or not, it is highly desirable to reduce the amount of mercuryemitted from coal burning utilities.

In a typical coal burning facility, raw coal arrives in railcars and isdelivered onto a receiving belt, which leads the coal into a pug mill.After the pug mill, the coal is discharged to a feed belt and depositedin a coal storage area. Under the coal storage area there is typically agrate and bin area; from there a belt transports the coal to an openstockpile area, sometimes called a bunker. Stoker furnaces can be fedwith coal from the bunker or from a crusher. For furnaces burningpulverized coal, the coal is delivered by belt or other means to millingequipment such as a crusher and ultimately to a pulverizer. In a storagesystem, coal is pulverized and conveyed by air or gas to a collector,from which the pulverized coal is transferred to a storage bin, fromwhich the coal is fed to the furnace as needed. In a direct firedsystem, coal is pulverized and transported directly to the furnace. In asemi-direct system, the coal goes from the pulverizer to a cyclonecollector. The coal is fed directly from the cyclone to the furnace.

During operation coal is fed into the furnace and burned in the presenceof oxygen. For high BTU fuels, typical flame temperatures in thecombustion chamber are on the order of 2700° F. (about 1480° C.) toabout 3000° F. (about 1640° C.) or even higher, such as 3300° F. (about1815° C.) to 3600° F. (about 1982° C.).

In various embodiments, sorbent compositions according to the inventionare added to the raw coal, in the pug mill, on the receiving belt orfeed belt, in the coal storage area, in the collector, in the storagebin, in the cyclone collector, in the pulverizer before or duringpulverization, and/or while being transported from the pulverizer to thefurnace for combustion. Conveniently, in various embodiments thesorbents are added to the coal during processes that mix the coal suchas the in the pug mill or in the pulverizer. In a preferred embodiment,the sorbents are added onto the coal in the pulverizers.

Alternatively or in addition, sorbent components are added into the coalburning system by injecting them into the furnace during combustion ofthe fuel. In a preferred embodiment, they are injected into the fireballor close to the fireball, for example where the temperature is above2000° F., above 2300° F., or above 2700° F. According to the design ofthe burners and the operating parameters of the furnace, effectivesorbent addition takes place along with the fuel, with the primarycombustion air, above the flame, with or above the overtire air, and soon. Also depending on the furnace design and operation, sorbents areinjected from one or more faces of the furnace and/or from one or morecorners of the furnace. Addition of sorbent compositions and sorbentcomponents tends to be most effective when the temperature at injectionis sufficiently high and/or the aerodynamics of the burners and furnaceset up lead to adequate mixing of the powder sorbents with the fueland/or combustion products. Alternatively or in addition, sorbentaddition is made to the convective pathway downstream of the flame andfurnace. In various embodiments, optimum injection or application pointsfor sorbents are found by modeling the furnace and choosing parameters(rate of injection, place of injection, distance above the flame,distance from the wall, mode of powder spraying, and the like) that givethe best mixing of sorbent, coal, and combustion products for thedesired results.

In coal burning systems, hot combustion gases and air move by convectionaway from the flame through the convective pathway in a downstreamdirection (i.e., downstream in relation to the fireball). The convectivepathway of the facility contains a number of zones characterized by thetemperature of the gases and combustion products in each zone.Generally, the temperature of the combustion gas falls as it moves in adirection downstream from the fireball. From the furnace, where the coalin one example is burning at a temperature of approximately 2700°F.-3600° F. (about 1480° C.-1650° C.), the fly ash and combustion gasesmove downstream in the convective pathway to zones of ever decreasingtemperature. To illustrate, downstream of the fireball is a zone withtemperature less that 2700° F. Further downstream, a point is reachedwhere the temperature has cooled to about 1500° F. Between the twopoints is a zone having a temperature from about 1500° F. to about 2700°F. Further downstream, a zone of less than 1500° F. is reached, and soon. Further along in the convective pathway, the gases and fly ash passthrough lower temperature zones until the baghouse or electrostaticprecipitator is reached, which typically has a temperature of about 300°F. before the gases are emitted up the stack.

The combustion gases contain carbon dioxide as well as variousundesirable gases containing sulfur and mercury. The convective pathwaysare also filled with a variety of ash which is swept along with the hightemperature gases. To remove the ash before emission into theatmosphere, particulate removal systems are used. A variety of suchremoval systems, such as electrostatic precipitators and a bag house,are generally disposed in the convective pathway. In addition, chemicalscrubbers can be positioned in the convective pathway. Additionally,there may be provided various instruments to monitor components of thegas such as sulfur and mercury.

Thus, in various embodiments, the process of the present invention callsfor the application of sorbents

directly into the furnace during combustion (addition “co-combustion”)

directly to a fuel such as coal before combustion (addition“pre-combustion”);

directly into the gaseous stream after combustion preferably in atemperature zone of greater than 500° C. and preferably greater than800° C. (addition “post-combustion”); or

in a combination of pre-combustion, co-combustion, and post-combustionadditions.

Application of the sorbents is made “into the coal burning system” inany of pre-combustion, co-combustion, or post-combustion modes, or inany combination. When the sorbents are added into the coal burningsystem, the coal or other fuel is said to be combusted “in the presence”the various sorbents, sorbent compositions, or sorbent components

In a preferred embodiment downstream addition is carried out where thetemperature is about 1500° F. (815.5° C.) to about 2700° F. (1482.2°C.). In some aspects, and depending upon the specifics of furnace designand the layout of the convective pathways, the cutoff point ordistinction between “into the furnace”, “into the fireball”, and “intothe convective pathways” can be rather arbitrary. At some point, thecombustion gases leave what is clearly a burning chamber or furnace andenter a separate structure that is clearly a flue or convective pathwayfor gases downstream of the furnace. However, many furnaces are quitelarge and so permit addition of sorbents “into the furnace” at aconsiderable distance from where the fuel and air are being fed to formthe fireball. For example, some furnaces have overfire air injectionports and the like specifically designed to provide additional oxygen ata location above the fireball to achieve more complete combustion and/orcontrol of emissions such as nitrogen oxides. The overfire air ports canbe 20 feet or higher above the fuel injection. In various embodiments,sorbent components or compositions are injected directly into thefireball along with coal being fed, at a location above the coal feedabove or below the overfire air ports, or at a higher location withinthe burning chamber, such as at or just under the nose of the furnace.Each of these locations is characterized by a temperature and byconditions of turbulent flow that contribute to mixing of the sorbentswith the fuel and/or the combustion products (such as the fly ash). Inembodiments involving applying sorbent compositions into the furnace ordownstream of the furnace, application is preferably made where thetemperature is above 1500° F., preferably above 2000° F., morepreferably where the temperature is above 2300° F., and most preferablywhere the temperature is above 2700° F.

In various embodiments, sorbents are added as coal is burned along withother fuels in co-generation plants. Such plants are flexible in thefuels they burn. In addition to bituminous and sub-bituminous coal, suchfacilities can also burn waste-derived fuels such without limitation asmunicipal waste, sewage sludge, pet coke, animal waste, plant waste(such as without limitation wood, rice hulls, wood chips, agriculturalwaste, and/or sawdust), scrap plastics, shredded tires, and the like. Tothe extent that the fuels contain mercury and sulfur, use of sorbents asdescribed herein tends to mitigate or lower emissions of sulfur and/ormercury that would otherwise be released into the atmosphere uponcombustion. Depending on the fuel value, the flame temperature in suchco-generation plants varies upward from about 1000° F.-1200° F. (for lowvalue fuels or fuels containing high proportions of low value biomass orother low-value components) to 2700° F. to 3600° F. or higher (for highBTU coal). In various embodiments, use of sorbents of the inventionmitigates mercury emissions from systems burning at relatively lowertemperatures. It is believed the sorbents are especially effective atremoving oxidized mercury from the flue gases, and that oxidized mercuryin the species predominantly formed by combustion at the lowertemperatures.

Thus, in various embodiments, co-generation plants burning a combinationof coal and a wide variety of other fuels (see above) are treated withsorbent compositions to achieve significant reductions in emissions ofmercury and/or sulfur.

In various embodiments described herein, sorbent compositions that tendto reduce or remediate the release of mercury and/or sulfur from coalburning utilities also have the beneficial effect of rendering the ashproduced by combustion of the fuel highly cementitious. As a result, theash is usable in commerce as a partial or complete replacement forportland cement in various cement and concrete products.

Burning the coal with the sorbent compositions described herein resultsin an ash that has, in various embodiments, increased levels of theheavy metals compared to coal burned without the sorbent, but whichnevertheless contains lower levels of leachable heavy metals than theash produced without the sorbents. As a result, the ash is safe tohandle and to sell into commerce, for example as a cementitiousmaterial.

To make the ash products, a carbonaceous fuel is burned to produce heatenergy from combustion of the carbonaceous material. Unburned materialand particulate combustion products form ash, some of which collects atthe bottom of the furnace, but the majority of which is collected as flyash from the flue by precipitators or filters, for example a bag houseon a coal burning facility. The content of the bottom ash and the flyash depends on the chemical composition of the coal and on the amountand composition of sorbent components added into the coal burningfacility during combustion.

In various embodiments, mercury emissions from the coal burning facilityare monitored. Emissions are monitored as elemental mercury, oxidizedmercury, or both. Elemental mercury means mercury in the ground or zerooxidation state, while oxidized mercury means mercury in the +1 or +2oxidation state. Depending on the level of mercury in the flue gas priorto emission from the plant, the amount of sorbent composition addedpre-, co-, and/or post-combustion is raised, lowered, or is maintainedunchanged. In general, it is desirable to remove as high a level ofmercury as is possible. In typical embodiments, mercury removal of 90%and greater is achieved, based on the total amount of mercury in thecoal. This number refers to the mercury removed from the flue gases sothat mercury is not released through the stack into the atmosphere.Normally, removal of mercury from the flue gases leads to increasedlevels of mercury in the ash. To minimize the amount of sorbent addedinto the coal burning process so as to reduce the overall amount of ashproduced in the furnace, it is desirable in many embodiments to use themeasurements of mercury emissions to adjust the sorbent composition rateof addition to one which will achieve the desired mercury reductionwithout adding excess material into the system.

In various embodiments of burning coal or other fuels with the addedsorbent components, mercury and other heavy metals in the coal such asarsenic, antimony, lead, and others report to the bag house orelectrostatic precipitator and become part of the overall ash content ofthe coal burning plant; alternatively or in addition, the mercury andheavy metals are found in the bottom ash. As such, mercury and otherheavy metals are not emitted from the facility. In general, mercury andother heavy metals in the ash are resistant to leaching under acidicconditions, even though they tend to be present in the ash at elevatedlevels relative to ash produced by burning coal without the sorbentcomponents described herein. Advantageously, heavy metals in the ash donot leach beyond regulatory levels; in fact, a decreased level ofleachable heavy metal is observed in the ash on a ppm basis, even thoughthe ash normally contains a higher absolute level of heavy metals byvirtue of being produced by burning with the sorbents. Because inaddition the cementitious nature of the ash is enhanced, the ash fromthe combustion (coal ash) is valuable for sale in commerce and use, forexample, as a cementitious material to make portland cements as well asconcrete products and ready mixes.

In preferred embodiments, leaching of heavy metals is monitored oranalyzed periodically or continuously during combustion. The TCLPprocedure of the United States Environmental Protection Agency is acommonly used method. The amount of sorbent, particularly of sorbentcomponents with Si (SiO₂ or equivalents) and/or Al (Al₂O₃ orequivalents), is adjusted based on the analytical result to maintain theleaching in a desired range.

In one embodiment, a method is provided for burning coal to reduce theamount of mercury released into the atmosphere. The method involvesapplying a sorbent composition comprising a halogen compound into thesystem in which the coal is being combusted. The halogen compound ispreferably a bromine compound; in a preferred embodiment, the sorbent isfree of alkali metal compounds so as to avoid corrosion on boiler tubesor other furnace components. The coal is combusted in the furnace toproduce ash and combustion gases. The combustion gases contain mercury,sulfur and other components. To accomplish a desired reduction ofmercury in the combustion gases in order to limit release into theatmosphere, the mercury level in the combustion gases is preferablymonitored, for example by measuring the level analytically. In preferredembodiments, the amount of the sorbent composition applied is adjusted(i.e., by increasing it, decreasing it, or in some cases deciding toleave it unchanged) depending on the value of the mercury level measuredin the combustion gases. In a preferred embodiment, the sorbent is addedinto the system by applying it to the coal pre-combustion, thendelivering the coal containing the sorbent into the furnace forcombustion.

In another embodiment, sorbent components comprising a halogen(preferably bromine or iodine, and most preferably bromine) compound andat least one aluminosilicate material are applied into the coal burningsystem. The components are added separately or as a single sorbentcomposition, and are optionally added onto the coal pre-combustion, intothe furnace during combustion, or into the flue gases downstream of thefurnace at suitable temperatures. In a preferred embodiment, thecomponents are added to the coal pre-combustion, and the coal containingthe sorbent is then delivered into the furnace for combustion. Asbefore, preferably mercury is monitored in the flue gases and thesorbent application rate is adjusted depending on the value of themeasured mercury level. The halogen contributes to lowering the level ofmercury emissions, while the aluminosilicate contributes to makingmercury captured in the ash non-leaching.

In a related embodiment, a method for reducing leaching of mercuryand/or of other heavy metals from ash produced from the combustion ofcoal or other fuel in a coal burning system or in an incineratorinvolves introducing sorbents containing silica and alumina into theincinerator or coal burning system during combustion, measuring leachingof mercury and/or other heavy metals from the resulting ash, andadjusting the level of silica and alumina added according to themeasured leaching of heavy metals. If leaching is higher than desired,the rate of application of the sorbent can be increased to bring theleaching back down into the desired range. In a preferred embodiment,the sorbent further contains a halogen (e.g. bromine) compound toenhance capture of mercury in the ash.

In one embodiment, the invention provides a method for reducing theamount of oxidized mercury in flue gases that are generated bycombustion of mercury-containing carbonaceous fuel such as coal while atthe same time producing a cementitious ash product. The method comprisesburning the fuel in the presence of an alkaline powder sorbent whereinthe powder sorbent comprises calcium, silica, and alumina. The alkalinepowder is added to the coal pre-combustion, injected into the furnaceduring combustion, applied into the flue gases downstream of the furnace(preferably where the temperature is 1500° F. or greater), or in anycombination. The powders are alkaline, characterized by a pH above 7when combined with water, preferably above 8 and preferably above 9.Preferably, the sorbent contains about 0.01 to about 5% by weight ofalkalis such as those based on Na₂O and K₂O. In various embodiments, thesorbent further contains iron and magnesium. In various embodiments, thealuminum content of the sorbent is higher than the alumina content ofportland cement, preferably above about 5% or above about 7% alumina.

While the fuel is burning, a level of mercury (oxidized, elemental, orboth) is measured in the flue gases downstream from the furnace. Themeasured mercury level is compared to a target level and, if themeasured level is above the target level, the amount of powder sorbentadded relative to the amount of fuel being burned is increased.Alternatively, if the measured level is at or below the target level,the rate of sorbent addition can be decreased or maintained unchanged.

In another embodiment, the powder composition is an alkaline sorbentcomposition that contains an alkaline calcium component as well assignificant levels of silica and alumina. In a non-limiting embodiment,the powder composition comprises 2 to 50% of an aluminosilicate materialand 50 to 98% by weight of an alkaline powder comprising calcium. In apreferred embodiment, the alkaline powder comprises one or more of lime,calcium oxide, portland cement, cement kiln dust, lime kiln dust, andsugar beet lime, while the aluminosilicate material contains one or moreselected from the group consisting of calcium montmorillonite, sodiummontmorillonite, and kaolin. The powder composition is added to the coalat a rate of about 0.1 to about 10% by weight, based on the amount ofcoal being treated with the sorbents for a batch process, or on the rateof coal being consumed by combustion for a continuous process. In apreferred embodiment, the rate is from 1 to 8% by weight, 2 to 8% byweight, 4 to 8% by weight, 4 to 6% by weight, or about 6% by weight. Inpreferred embodiments, the powder composition is injected to thefireball or furnace during combustion and/or is applied to the coalunder ambient conditions, prior to its combustion. The temperature atthe injection point is preferably at least about 1000° F. or higher. Forsome low value fuels, this corresponds to injection into or close to thefireball.

In another embodiment, the invention provides novel sorbent compositionscomprising about 50 to 98% by weight of at least one of portland cement,cement kiln dust, lime kiln dust, sugar beet lime, and 2 to 50% byweight of an aluminosilicate material. In a various embodiments, thecompositions further comprise a bromine compound, for example a bromidesuch as calcium bromide. Use of the sorbents during the coal burningprocess as described herein tends to lessen the amount of harmful sulfurand mercury products emitted from the facility, while at the same timeproducing an ash that is environmentally acceptable (e.g. leaching ofheavy metals is below regulatory levels and is lower than in ashproduced by burning the coal without the sorbent components) and highlycementitious in nature so that the ash serves as a complete or partial(greater than 40%, preferably greater than 50%) replacement for portlandcement in cementitious mixtures and processes for their use.

In yet another embodiment, a method is provided for burning a fuelcontaining mercury and optionally sulfur so that the level of harmfulcompounds emitted in the combustion gases and released into theenvironment is reduced. In a preferred embodiment, the method involvesapplying a sorbent onto the fuel and burning the fuel containing thesorbent to produce gases and fly ash. The sorbent contains bromine,calcium, silica, and alumina.

In a further embodiment, a method for reducing mercury and/or sulfuremitted into the environment during combustion of coal in a coal burningsystem comprises adding sorbent components comprising bromine, calcium,silica, and alumina into the coal burning system and combusting the coalin the presence of the sorbent components to produce combustion gasesand fly ash. The amount of mercury in the combustion gases is measuredand level of components containing bromine added into the system isadjusted depending on the measured value of mercury in the combustiongases.

In various embodiments, the four components (calcium, silica, alumina,and bromine) are added together or separately to the coalpre-combustion, to the furnace, and/or to the flue gases at suitabletemperature as described herein. Preferably, bromine is present at alevel effective to capture, in the ash, at least 90% of the mercury inthe coal, and silica and alumina are present at levels effective toproduce fly ash with a leaching value of less than 0.2 ppm (200 ppb)with respect to mercury, preferably less than 100 ppb Hg, less than 50ppb, and most preferably less than 2 ppb with respect to mercury. Alevel of 2 ppb represents the current lower detectable limit of the TCLPtest for mercury leaching.

In a particular embodiment, a dual sorbent system is used whereincalcium, silica, and alumina are added in a powder sorbent, whilebromine or other halogen(s) is added in a liquid sorbent. The liquid andpowder sorbents are added into the coal burning system onto the coalpre-combustion, into the furnace, into the flue gases (at suitabletemperatures as described herein), or in any combination. In a preferredembodiment, liquid sorbent is added to the coal pre-combustion andpowder sorbent is added either to the coal pre-combustion or to thefurnace during combustion. Treatment levels of the liquid and powdersorbents, as well as preferred compositions, are described herein.

In preferred embodiments, the methods provide coal ash and/or fly ashcontaining mercury at a level corresponding to capture in the ash of atleast 90% of the mercury originally in the coal before combustion. Insome embodiments, the mercury level is higher than in known fly ashesdue to capture of mercury in the ash rather than release of mercury intothe atmosphere. Fly ash produced by the process contains up to 200 ppmmercury or higher; in some embodiments the mercury content of the flyash is above 250 ppm. Since the volume of ash is normally increased byuse of the sorbents (in typical embodiments, the volume of ash aboutdoubles), the increased measured levels of mercury represent significantcapture in the ash of mercury that without the sorbents would have beenreleased into the environment. The content in the fly ash of mercury andother heavy metals such as lead, chromium, arsenic, and cadmium isgenerally higher than in fly ash produced from burning coal without theadded sorbents or sorbent components.

In various embodiments, an ash product is produced by burning coal inthe presence of sorbent components comprising calcium, silica, alumina,and preferably a halogen such as bromine. The components are added asparts of one or more sorbent compositions into the coal-burning system.In a non-limiting example, sorbent components calcium, silica, andalumina are added together in an alkaline powder sorbent compositionthat comprises about 2 to 15% by weight Al₂O₃, about 30 to 75% by weightCaO, about 5 to 20% by weight SiO₂, about 1 to 10% Fe₂O₃, and about 0.1to 5% by weight total alkali, such as sodium oxide and potassium oxide.In a preferred embodiment, the sorbents comprise about 2 to 10% byweight Al₂O₃, about 40 to 70% by weight CaO, and about 5 to 15% byweight SiO₂ in addition to the total alkalis. In a preferred embodiment,powder sorbent compositions described herein contain one or morealkaline powders containing calcium, along with lesser levels of one ormore aluminosilicate materials. The halogen component, if desired, isadded as a further component of the alkaline powder or is addedseparately as part of a liquid or powder composition. Advantageously,use of the sorbents leads to a reduction in the levels of sulfur,mercury, other heavy metals such as lead and arsenic, and/or chlorinefrom the coal burning system.

In another embodiment, the method of the invention provides coal ashcontaining mercury at a level corresponding to capture in the ash of atleast 90% of the mercury originally in the coal before combustion. Aprocess for making the coal ash involves burning coal in the presence ofadded calcium, silica, and alumina, and preferably in the furtherpresence of a halogen such as bromine. In a preferred embodiment, ash isprepared by burning coal in the presence of sorbents or sorbentcomponents described herein. Preferably, the mercury in the coal ash isnon-leaching in that it exhibits a concentration of mercury in theextract of less than 0.2 ppm when tested using the ToxicityCharacteristic Leaching Procedure (TCLP), test Method 1311 in “TestMethods for Evaluating Solid Waste, Physical/Chemical Methods,” EPAPublication SW-846-Third Edition, as incorporated by reference in 40 CFR§260.11. It is normally observed that fly ash from burning coal with thesorbents described herein has less leachable mercury than ash producedfrom burning coal without the sorbent, even though the total mercurycontent in ash produced from the sorbent treated coal is higher by asmuch as a factor of 2 or more over the level in ash produced by burningwithout the sorbents. To illustrate, typical ash from burning of PRBcoal contains about 100-125 ppm mercury; in various embodiments, ashproduced by burning PRB coal with about 6% by weight of the sorbentsdescribed herein has about 200-250 ppm mercury or more.

In another embodiment, the invention provides a hydraulic cement productcontaining portland cement and from 0.1% to about 99% by weight, basedon the total weight of the cement product, of a coal ash or fly ashdescribed above.

In a further embodiment, the invention provides a pozzolanic productcomprising a pozzolan and from 0.01% to about 99% by weight, based onthe total weight of the pozzolanic product of the ash described above.

The invention also provides a cementitious mixture containing thehydraulic cement product.

The invention further provides a concrete ready mix product containingaggregate and the hydraulic cement product.

In another embodiment, a cementitious mixture contains coal ashdescribed herein as the sole cementitious component; in theseembodiments, the ash is a total replacement for conventional cementssuch as portland cement. The cementitious mixtures contain cement andoptionally aggregate, fillers, and/or other admixtures. The cementitiousmixtures are normally combined with water and used as concrete, mortars,grout, flowable fill, stabilized base, and other applications.

The methods thus encompass burning coal with the added sorbents toproduce coal ash and energy for heat or electricity generation. The ashis then recovered and used to formulate cementitious mixtures includingcements, mortars, and grouts.

Sorbent compositions used in various embodiments of the inventiondescribed above and herein contain components that contribute calcium,silica, and/or alumina, preferably in the form of alkaline powders. Invarious embodiments, the compositions also contain iron oxide, as wellas basic powders based on sodium oxide (Na₂O) and potassium oxide (K₂O).In a non-limiting example, the powder sorbent contains about 2-10% byweight Al₂O₃, about 40-70% CaO, about 5-15% SiO₂, about 2-9% Fe₂O₃, andabout 0.1-5% total alkalis such as sodium oxide and potassium oxide. Thecomponents comprising calcium, silica, and alumina—and other elements ifpresent—are combined together in a single composition or are addedseparately or in any combination as components to the fuel burningsystem. In preferred embodiments, use of the sorbents leads toreductions in the amount of sulfur and/or mercury released into theatmosphere. In various embodiments, use of the sorbent compositionsleads to the removal of mercury, especially oxidized mercury. Inaddition, the compositions reduce the amount of sulfur given off fromcombustion by a virtue of their calcium content.

Advantageously, the sorbent compositions contain suitable high levels ofalumina and silica. It is believed that the presence of alumina and/orsilica leads to several advantages seen from use of the sorbent. Toillustrate, it is believed that the presence of alumina and/or silicaand/or the balance of the silica/alumina with calcium, iron, and otheringredients contributes to the low acid leaching of mercury and/or otherheavy metals that is observed in ash produced by combustion of coal orother fuels containing mercury in the presence of the sorbents.

In various embodiments, it is observed that use of the sorbentcompositions during combustion of coal or other fuel leads to theformation of a refractory lining on the walls of the furnace and on theboiler tubes. It is believed that such a refractory lining reflects heatin the furnace and leads to higher water temperature in the boilers. Invarious embodiments, it is also observed that use of the sorbent resultsin reduced scale formation or slagging around the boiler tubes. In thisway, use of the sorbents leads to cleaner furnaces, but more importantlyimproves the heat exchange between the burning coal and the water in theboiler tubes. As a result, in various embodiments use of the sorbentsleads to higher water temperature in the boiler, based on burning thesame amount of fuel. Alternatively, it has been observed that use of thesorbent allows the feed rate of, for example, coal to be reduced whilemaintaining the same power output or boiler water temperature. In anillustrative embodiment, use of a sorbent at a 6% rate results incombustion of a coal/sorbent composition that produces as much power asa composition of the same weight that is all coal. It is seen in suchembodiments that use of the sorbent, which is normally captured in thefly ash and recycled, actually increases the efficiency of the coalburning process, leading to less consumption of fuel. Advantageously insuch a process, the fly ash, which is normally increased in volume byvirtue of the use of the sorbent, is recycled for use in portland cementmanufacture and the like, it having an improved cementitious nature andlow heavy metal leaching.

As noted, the components that contribute calcium, silica, and/or aluminaare preferably provided as alkaline powders. Without being limited bytheory, it is believed that the alkaline nature of the sorbentcomponents leads at least in part to the desirable properties describedabove. For example, it is believed the alkaline nature of the powdersleads to a reduction in sulfur pitting. After neutralization, it isbelieved a geopolymeric ash is formed in the presence of the sorbents,coupling with silica and alumina present in the sorbent to form aceramic like matrix that reports as a stabilized ash. The stabilized ashis characterized by very lowing leaching of mercury and other heavymetals. In some embodiments, the leaching of mercury is below detectablelimits.

Sources of calcium for the sorbent compositions of the inventioninclude, without limitation, calcium powders such as calcium carbonate,limestone, dolomite, calcium oxide, calcium hydroxide, calciumphosphate, and other calcium salts. Industrial products such aslimestone, lime, slaked lime, and the like contribute major proportionsof such calcium salts. As such, they are suitable components for thesorbent compositions of the invention.

Other sources of calcium include various manufactured products. Suchproducts are commercially available, and some are sold as waste productsor by-products of other industrial processes. In preferred embodiments,the products further contribute either silica, alumina, or both to thecompositions of the invention. Non-limiting examples of industrialproducts that contain silica and/or alumina in addition to calciuminclude portland cement, cement kiln dust, lime kiln dust, sugar beetlime, slags (such as steel slag, stainless steel slag, and blast furnaceslag), paper de-inking sludge ash, cupola arrester filter cake, andcupola furnace dust.

These and optionally other materials are combined to provide alkalinepowders or mixtures of alkaline powders that contain calcium, andpreferably also contain silica and alumina. Other alkaline powderscontaining calcium, silica, and alumina include pozzolanic materials,wood ash, rice hull ash, class C fly ash, and class F fly ash. Invarious embodiments, these and similar materials are suitable componentsof sorbent compositions, especially if the resulting compositioncontaining them as components falls within the preferred range of 2 to15% by weight Al₂O₃, about 30 to 75% by weight CaO, about 5 to 20% byweight SiO₂, about 1 to 10% Fe₂O₃, and about 0.1 to 5% by weight totalalkali. Mixtures of materials are also used. Non-limiting examplesinclude mixtures of portland cement and lime, and mixtures containingcement kiln dust, such as cement kiln dust and lime kiln dust.

Sugar beet lime is a solid waste material resulting from the manufactureof sugar from sugar beets. It is high in calcium content, and alsocontains various impurities that precipitate in the liming procedurecarried out on sugar beets. It is an item of commerce, and is normallysold to landscapers, farmers, and the like as a soil amendment.

Cement kiln dust (CKD) generally refers to a byproduct generated withina cement kiln or related processing equipment during portland cementmanufacturing.

Generally, CKD comprises a combination of different particles generatedin different areas of the kiln, pre-treatment equipment, and/or materialhandling systems, including for example, clinker dust, partially tofully calcined material dust, and raw material (hydrated and dehydrated)dust. The composition of the CKD varies based upon the raw materials andfuels used, the manufacturing and processing conditions, and thelocation of collection points for CKD within the cement manufacturingprocess. CKD can include dust or particulate matter collected from kilneffluent (i.e., exhaust) streams, clinker cooler effluent, pre-calcinereffluent, air pollution control devices, and the like.

While CKD compositions will vary for different kilns, CKD usually has atleast some cementitious and/or pozzolanic properties, due to thepresence of the dust of clinker and calcined materials. Typical CKDcompositions comprise silicon-containing compounds, such as silicatesincluding tricalcium silicate, dicalcium silicate; aluminum-containingcompounds, such as aluminates including tricalcium aluminate; andiron-containing compounds, such as ferrites including tetracalciumaluminoferrite. CKD generally comprises calcium oxide (CaO). ExemplaryCKD compositions comprise about 10 to about 60% calcium oxide,optionally about 25 to about 50%, and optionally about 30 to about 45%by weight. In some embodiments, CKD comprises a concentration of freelime (available for a hydration reaction with water) of about 1 to about10%, optionally of about 1 to about 5%, and in some embodiments about 3to about 5%. Further, in certain embodiments, CKD comprises compoundscontaining alkali metals, alkaline earth metals, and sulfur, inter alia.

Other exemplary sources for the alkaline powders comprising calcium, andpreferably further comprising silica and alumina, include variouscement-related byproducts (in addition to portland cement and CKDdescribed above). Blended-cement products are one suitable example ofsuch a source. These blended cement products typically contain mixes ofportland cement and/or its clinker combined with slag(s) and/orpozzolan(s) (e.g., fly ash, silica fume, burned shale). Pozzolans areusually silicaceous materials that are not in themselves cementitious,but which develop hydraulic cement properties when reacted with freelime (free CaO) and water. Other sources are masonry cement and/orhydraulic lime, which include mixtures of portland cement and/or itsclinker with lime or limestone. Other suitable sources are aluminouscements, which are hydraulic cements manufactured by burning a mix oflimestone and bauxite (a naturally occurring, heterogeneous materialcomprising one or more aluminum hydroxide minerals, plus variousmixtures of silica, iron oxide, titania, aluminum silicates, and otherimpurities in minor or trace amounts). Yet another example is a pozzolancement, which is a blended cement containing a substantial concentrationof pozzolans. Usually the pozzolan cement comprises calcium oxide, butis substantially free of portland cement. Common examples ofwidely-employed pozzolans include natural pozzolans (such as certainvolcanic ashes or tuffs, certain diatomaceous earth, burned clays andshales) and synthetic pozzolans (such as silica fume and fly ash).

Lime kiln dust (LKD) is a byproduct from the manufacturing of lime. LKDis dust or particulate matter collected from a lime kiln or associatedprocessing equipment. Manufactured lime can be categorized ashigh-calcium lime or dolomitic lime, and LKD varies based upon theprocesses that form it. Lime is often produced by a calcination reactionconducted by heating calcitic raw material, such as calcium carbonate(CaCO₃), to form free lime CaO and carbon dioxide (CO₂). High-calciumlime has a high concentration of calcium oxide and typically someimpurities, including aluminum-containing and iron-containing compounds.High-calcium lime is typically formed from high purity calcium carbonate(about 95% purity or greater). Typical calcium oxide content in an LKDproduct derived from high-calcium lime processing is greater than orequal to about 75% by weight, optionally greater than or equal to about85% by weight, and in some cases greater than or equal to about 90% byweight. In some lime manufacturing, dolomite (CaCO₃.MgCO₃) is decomposedby heating to primarily generate calcium oxide (CaO) and magnesium oxide(MgO), thus forming what is known as dolomitic lime. In LKD generated bydolomitic lime processing, calcium oxide can be present at greater thanor equal to about 45% by weight, optionally greater than about 50% byweight, and in certain embodiments, greater than about 55% by weight.While LKD varies based upon the type of lime processing employed, itgenerally has a relatively high concentration of free lime. Typicalamounts of free lime in LKD are about 10 to about 50%, optionally about20 to about 40%, depending upon the relative concentration of calciumoxide present in the lime product generated.

Slags are generally byproduct compounds generated by metal manufacturingand processing. The term “slag” encompasses a wide variety of byproductcompounds, typically comprising a large portion of the non-metallicbyproducts of ferrous metal and/or steel manufacturing and processing.Generally, slags are considered to be a mixture of various metal oxides,however they often contain metal sulfides and metal atoms in anelemental form.

Various examples of slag byproducts useful for certain embodiments ofthe invention include ferrous slags, such as those generated in blastfurnaces (also known as cupola furnaces), including, by way of example,air-cooled blast furnace slag (ACBFS), expanded or foamed blast furnaceslag, pelletized blast furnace slag, granulated blast furnace slag(GBFS), and the like. Steel slags can be produced from basic oxygensteelmaking furnaces (BOS/BOF) or electric arc furnaces (EAF). Manyslags are recognized for having cementitious and/or pozzolanicproperties, however the extent to which slags have these propertiesdepends upon their respective composition and the process from whichthey are derived, as recognized by the skilled artisan. Exemplary slagscomprise calcium-containing compounds, silicon-containing compounds,aluminum-containing compounds, magnesium-containing compounds,iron-containing compounds, manganese-containing compounds and/orsulfur-containing compounds. In certain embodiments, the slag comprisescalcium oxide at about 25 to about 60%, optionally about 30 to about50%, and optionally about 30 to about 45% by weight. One example of asuitable slag generally having cementitious properties is groundgranulated blast furnace slag (GGBFS).

As described above, other suitable examples include blast (cupola)furnace dust collected from air pollution control devices attached toblast furnaces, such as cupola arrester filter cake. Another suitableindustrial byproduct source is paper de-inking sludge ash. As recognizedby those of skill in the art, there are many differentmanufactured/industrial process byproducts that are feasible as a sourceof calcium for the alkaline powders that form the sorbent compositionsof the invention. Many of these well known byproducts comprise aluminaand/or silica, as well. Some, such as lime kiln dust, contain majoramounts of CaO and relatively small amounts of silica and alumina.Combinations of any of the exemplary manufactured products and/orindustrial byproducts are also contemplated for use as the alkalinepowders of certain embodiments of the invention.

In various embodiments, desired treat levels of silica and/or aluminaare above those provided by adding materials such as portland cement,cement kiln dust, lime kiln dust, and/or sugar beet lime. Accordingly,it is possible to supplement such materials with aluminosilicatematerials, such as without limitation clays (e.g. montmorillonite,kaolins, and the like) where needed to provide preferred silica andalumina levels. In various embodiments, supplemental aluminosilicatematerials make up at least about 2%, and preferably at least about 5% byweight of the various sorbent components added into the coal burningsystem. In general, there is no upper limit from a technical point ofview as long as adequate levels of calcium are maintained. However, froma cost standpoint, it is normally desirable to limit the proportion ofmore expensive aluminosilicate materials. Thus, the sorbent componentspreferably comprise from about 2 to 50%, preferably 2 to 20%, and morepreferably, about 2 to 10% by weight aluminosilicate material such asthe exemplary clays. A non-limiting example of a sorbent is about 93% byweight of a blend of CKD and LKD (for example, a 50:50 blend or mixture)and about 7% by weight of an aluminosilicate clay.

In various embodiments, an alkaline powder sorbent composition containsone or more calcium-containing powders such as portland cement, cementkiln dust, lime kiln dust, various slags, and sugar beet lime, alongwith an aluminosilicate clay such as, without limitation,montmorillonite or kaolin. The sorbent composition preferably containssufficient SiO₂ and Al₂O₃ to form a refractory-like mixture with calciumsulfate produced by combustion of the sulfur-containing coal in thepresence of the CaO sorbent component such that the calcium sulfate ishandled by the particle control system; and to form a refractory mixturewith mercury and other heavy metals so that the mercury and other heavymetals are not leached from the ash under acidic conditions. Inpreferred embodiments, the calcium containing powder sorbent contains byweight a minimum of 2% silica and 2% alumina, preferably a minimum of 5%silica and 5% alumina. Preferably, the alumina level is higher than thatfound in portland cement, that is to say higher than about 5% by weight,preferably higher than about 6% by weight, based on Al₂O₃.

In various embodiments, the sorbent components of the alkaline powdersorbent composition work together with optional added halogen (such asbromine) compound or compounds to capture chloride as well as mercury,lead, arsenic, and other heavy metals in the ash, render the heavymetals non-leaching under acidic conditions, and improve thecementitious nature of the ash produced. As a result, emissions ofharmful elements are mitigated, reduced, or eliminated, and a valuablecementitious material is produced as a by-product of coal burning.

Suitable aluminosilicate materials include a wide variety of inorganicminerals and materials. For example, a number of minerals, naturalmaterials, and synthetic materials contain silicon and aluminumassociated with an oxy environment along with optional other cationssuch as, without limitation, Na, K, Be, Mg, Ca, Zr, V, Zn, Fe, Mn,and/or other anions, such as hydroxide, sulfate, chloride, carbonate,along with optional waters of hydration. Such natural and syntheticmaterials are referred to herein as aluminosilicate materials and areexemplified in a non-limiting way by the clays noted above.

In aluminosilicate materials, the silicon tends to be present astetrahedra, while the aluminum is present as tetrahedra, octahedra, or acombination of both. Chains or networks of aluminosilicate are built upin such materials by the sharing of 1, 2, or 3 oxygen atoms betweensilicon and aluminum tetrahedra or octahedra. Such minerals go by avariety of names, such as silica, alumina, aluminosilicates, geopolymer,silicates, and aluminates. However presented, compounds containingaluminum and/or silicon tend to produce silica and alumina upon exposureto high temperatures of combustion in the presence of oxygen

In one embodiment, aluminosilicate materials include polymorphs ofSiO₂.Al₂O₃. For example, silliminate contains silica octahedra andalumina evenly divided between tetrahedra and octahedra. Kyanite isbased on silica tetrahedra and alumina octahedra. Andalusite is anotherpolymorph of SiO₂.Al₂O₃.

In other embodiments, chain silicates contribute silicon (as silica)and/or aluminum (as alumina) to the compositions of the invention. Chainsilicates include without limitation pyroxene and pyroxenoid silicatesmade of infinite chains of SiO₄ tetrahedra linked by sharing oxygenatoms.

Other suitable aluminosilicate materials include sheet materials suchas, without limitation, micas, clays, chrysotiles (such as asbestos),talc, soapstone, pyrophillite, and kaolinite. Such materials arecharacterized by having layer structures wherein silica and aluminaoctahedra and tetrahedra share two oxygen atoms. Layeredaluminosilicates include clays such as chlorites, glauconite, illite,polygorskite, pyrophillite, sauconite, vermiculite, kaolinite, calciummontmorillonite, sodium montmorillonite, and bentonite. Other examplesinclude micas and talc.

Suitable aluminosilicate materials also include synthetic and naturalzeolites, such as without limitation the analcime, sodalite, chabazite,natrolite, phillipsite, and mordenite groups. Other zeolite mineralsinclude heulandite, brewsterite, epistilbite, stilbite, yagawaralite,laumontite, ferrierite, paulingite, and clinoptilolite. The zeolites areminerals or synthetic materials characterized by an aluminosilicatetetrahedral framework, ion exchangeable “large cations” (such as Na, K,Ca, Ba, and Sr) and loosely held water molecules.

In other embodiments, framework or 3D silicates, aluminates, andaluminosilicates are used. Framework aluminosilicates are characterizedby a structure where SiO₄ tetrahedra, AlO₄ tetrahedra, and/or AlO₆octahedra are linked in three dimensions. Non-limiting examples offramework silicates containing both silica and alumina include feldsparssuch as albite, anorthite, andesine, bytownite, labradorite, microcline,sanidine, and orthoclase.

In one aspect, the sorbent powder compositions are characterized in thatthey contain a major amount of calcium, preferably greater than 20% byweight based on calcium oxide, and that furthermore they contain levelsof silica, and/or alumina higher than that found in commercial productssuch as portland cement. In preferred embodiments, the sorbentcompositions comprise greater than 5% by weight alumina, preferablygreater than 6% by weight alumina, preferably greater than 7% by weightalumina, and preferably greater than about 8% by weight alumina.

Coal or other fuel is treated with sorbent components at rates effectiveto control the amount of sulfur and/or mercury released into theatmosphere upon combustion. In various embodiments, total treatmentlevels of the sorbent components ranges from about 0.1% to about 20% byweight, based on the weight of the coal being treated or on the rate ofthe coal being consumed by combustion, when the sorbent is a powdersorbent containing calcium, silica, and alumina. When the sorbentcomponents are combined into a single composition, the component treatlevels correspond to sorbent treat levels. In this way a single sorbentcomposition can be provided and metered or otherwise measured foraddition into the coal burning system. In general, it is desirable touse a minimum amount of sorbent so as not to overload the system withexcess ash, while still providing enough to have a desired effect onsulfur and/or mercury emissions. Accordingly, in preferred embodiments,the treatment level of sorbent ranges from about 1% to about 10% byweight, and preferably from about 1 or 2% by weight to about 10% byweight. For many coals, an addition rate of 6% by weight of powdersorbent has been found to be acceptable.

The powder sorbents containing calcium, silica, and alumina as describedherein are generally effective to reduce the amount of sulfur in gasesemitted from the coal burning facility. For reduction of sulfuremissions, it is preferred to provide calcium in the sorbent componentsat a molar ratio of at least 1:1, and preferably above 1:1, measuredagainst the moles of sulfur in the fuel (such as coal) being burned. Ifit is desired to avoid production of excess ash, the amount of calciumdelivered by way of the sorbent can be limited to, say, a maximum molarratio of 3:1, again measured against sulfur in the coal.

In some embodiments, the amount of mercury released is also mitigated,lowered, or eliminated by use of such sorbents even without additionalhalogen. It is believed that the sorbents are effective at removingoxidized mercury in systems where the flame temperature is as low as1000° F. However, in many embodiments, including some in which the flametemperature is considerably higher than 1000° F., it is preferable totreat the coal with sorbent compositions that contain a halogencompound. The use of the halogen compound along with the alkaline powdersorbent tends to reduce the amount of unoxidized mercury in the gases ofcombustion.

Sorbent compositions comprising a halogen compound contain one or moreorganic or inorganic compounds that contain a halogen. Halogens includechlorine, bromine, and iodine. Preferred halogens are bromine andiodine. The halogen compounds are sources of the halogens, especially ofbromine and iodine. For bromine, sources of the halogen include variousinorganic salts of bromine including bromides, bromates, andhypobromites. In various embodiments, organic bromine compounds are lesspreferred because of their cost or availability. However, organicsources of bromine containing a suitably high level of bromine areconsidered within the scope of the invention. Non-limiting examples oforganic bromine compounds include methylene bromide, ethyl bromide,bromoform, and carbon tetrabromide. Non-limiting inorganic sources ofiodine include hypoiodites, iodates, and iodides, with iodides beingpreferred. Organic iodine compounds can also be used.

When the halogen compound is an inorganic substituent, it is preferablya bromine or iodine containing salt of an alkaline earth element.Exemplary alkaline earth elements include beryllium, magnesium, andcalcium. Of halogen compounds, particularly preferred are bromides andiodides of alkaline earth metals such as calcium. Alkali metal bromineand iodine compounds such as bromides and iodides are effective inreducing mercury emissions. But in some embodiments, they are lesspreferred as they tend to cause corrosion on the boiler tubes and othersteel surfaces and/or contribute to tube degradation and/or firebrickdegradation. In various embodiments, it has been found desirable toavoid potassium salts of the halogens, in order to avoid problems in thefurnace.

In various embodiments, it has been found that the use of alkaline earthsalts such as calcium tends to avoid such problems with sodium and/orpotassium. Thus in various embodiments, the sorbents added into the coalburning system contain essentially no alkali metal-containing bromine oriodine compounds, or more specifically essentially no sodium-containingor potassium-containing bromine or iodine compounds.

In various embodiments, sorbent compositions containing halogen areprovided in the form of a liquid or of a solid composition. In variousembodiments, the halogen-containing composition is applied to the coalbefore combustion, is added to the furnace during combustion, and/or isapplied into flue gases downstream of the furnace. When the halogencomposition is a solid, it can further contain the calcium, silica, andalumina components described herein as the powder sorbent.Alternatively, a solid halogen composition is applied onto the coaland/or elsewhere into the combustion system separately from the sorbentcomponents comprising calcium, silica, and alumina. When it is a liquidcomposition it is generally applied separately.

In various embodiments, liquid mercury sorbent comprises a solutioncontaining 5 to 60% by weight of a soluble bromine or iodine containingsalt. Non-limiting examples of preferred bromine and iodine saltsinclude calcium bromide and calcium iodide. In various embodiments,liquid sorbents contain 5-60% by weight of calcium bromide and/orcalcium iodide. For efficiency of addition to the coal prior tocombustion, in various embodiments it is preferred to add mercurysorbents having as high level of bromine or iodine compound as isfeasible. In a non-limiting embodiment, the liquid sorbent contains 50%or more by weight of the halogen compound, such as calcium bromide orcalcium iodide.

In various embodiments, the sorbent compositions containing a halogencompound further contain a nitrate compound, a nitrite compound, or acombination of nitrate and nitrite compounds. Preferred nitrate andnitrite compounds include those of magnesium and calcium, preferablycalcium.

To further illustrate, one embodiment of the present invention involvesthe addition of liquid mercury sorbent directly to raw or crushed coalprior to combustion. For example, mercury sorbent is added to the coalin the coal feeders. Addition of liquid mercury sorbent ranges from 0.01to 5%. In various embodiments, treatment is at less than 5%, less than4%, less than 3%, or less than 2%, where all percentages are based onthe amount of coal being treated or on the rate of coal consumption bycombustion. Higher treatment levels are possible, but tend to wastematerial, as no further benefit is achieved. Preferred treatment levelsare from 0.025 to 2.5% by weight on a wet basis. The amount of solidbromide or iodide salt added by way of the liquid sorbent is of coursereduced by its weight fraction in the sorbent. In an illustrativeembodiment, addition of bromide or iodide compound is at a low levelsuch as from 0.01% to 1% by weight based on the solid. When a 50% byweight solution is used, the sorbent is then added at a rate of 0.02% to2% to achieve the low levels of addition. For example, in a preferredembodiment, the coal is treated by a liquid sorbent at a rate of 0.02 to1%, preferably 0.02 to 0.5% calculated assuming the calcium bromide isabout 50% by weight of the sorbent. In a typical embodiment,approximately 1%, 0.5%, or 0.25% of liquid sorbent containing 50%calcium bromide is added onto the coal prior to combustion, thepercentage being based on the weight of the coal. In a preferredembodiment, initial treatment is started at low levels (such as 0.01% to0.1%) and is incrementally increased until a desired (low) level ofmercury emissions is achieved, based on monitoring of emissions. Similartreatment levels of halogen are used when the halogen is added as asolid or in multi-component compositions with other components such ascalcium, silica, alumina, iron oxide, and so on.

When used, liquid sorbent is sprayed, dripped, or otherwise deliveredonto the coal or elsewhere into the coal burning system. In variousembodiments, addition is made to the coal or other fuel at ambientconditions prior to entry of the fuel/sorbent composition into thefurnace. For example, sorbent is added onto powdered coal prior to itsinjection into the furnace. Alternatively or in addition, liquid sorbentis added into the furnace during combustion and/or into the flue gasesdownstream of the furnace. Addition of the halogen containing mercurysorbent composition is often accompanied by a drop in the mercury levelsmeasured in the flue gases within a minute or a few minutes; in variousembodiments, the reduction of mercury is in addition to a reductionachieved by use of an alkaline powder sorbent based on calcium, silica,and alumina.

In another embodiment, the invention involves the addition of a halogencomponent (illustratively a calcium bromide solution) directly to thefurnace during combustion. In another embodiment, the invention providesfor an addition of a calcium bromide solution such as discussed above,into the gaseous stream downstream of the furnace in a zonecharacterized by a temperature in the range of 2700° F. to 1500° F.,preferably 2200° F. to 1500° F. In various embodiments, treat levels ofbromine compounds, such as calcium bromide are divided between co-, pre-and post-combustion addition in any proportion.

In one embodiment, various sorbent components are added onto coal priorto its combustion. The coal is preferably particulate coal, and isoptionally pulverized or powdered according to conventional procedures.In a non-limiting example, the coal is pulverized so that 75% by weightof the particles passes through a 200 mesh screen (a 200 mesh screen hashole diameters of 75 μm). In various embodiments, the sorbent componentsare added onto the coal as a solid or as a combination of a liquid and asolid. Generally, solid sorbent compositions are in the form of apowder. If a sorbent is added as a liquid (illustratively as a solutionof one or more bromine or iodine salts in water), in one embodiment thecoal remains wet when fed into the burner. In various embodiments, asorbent composition is added onto the coal continuously at the coalburning facility by spraying or mixing onto the coal while it is on aconveyor, screw extruder, or other feeding apparatus. In addition oralternatively, a sorbent composition is separately mixed with the coalat the coal burning facility or at the coal producer. In a preferredembodiment, the sorbent composition is added as a liquid or a powder tothe coal as it is being fed into the burner. For example, in a preferredcommercial embodiment, the sorbent is applied into the pulverizers thatpulverize the coal prior to injection. If desired, the rate of additionof the sorbent composition is varied to achieve a desired level ofmercury emissions. In one embodiment, the level of mercury in the fluegases is monitored and the level of sorbent addition adjusted up or downas required to maintain the desired mercury level.

In various embodiments, levels of mercury and/or sulfur emitted from thefacility are monitored with conventional analytical equipment usingindustry standard detection and determination methods. In oneembodiment, monitoring is conducted periodically, either manually orautomatically. In a non-limiting example, mercury emissions aremonitored once an hour to ensure compliance with government regulations.To illustrate, the Ontario Hydro method is used. In this known method,gases are collected for a pre-determined time, for example one hour.Mercury is precipitated from the collected gases, and the level ofelemental and/or oxidized mercury is quantitated using a suitable methodsuch as atomic absorption. Monitoring can also take more or lessfrequently than once an hour, depending on technical and commercialfeasibility. Commercial continuous mercury monitors can be set tomeasure mercury and produce a number at a suitable frequency, forexample once every 3 to 7 minutes. In various embodiments, the output ofthe mercury monitors is used to control the rate of addition of mercurysorbent. Depending on the results of monitoring, the rate of addition ofthe mercury sorbent is adjusted by either increasing the level ofaddition; decreasing it; or leaving it unchanged. To illustrate, ifmonitoring indicates mercury levels are higher than desired, the rate ofaddition of sorbent is increased until mercury levels return to adesired level. If mercury levels are at desired levels, the rate ofsorbent addition can remain unchanged. Alternatively, the rate ofsorbent addition can be lowered until monitoring indicates it should beincreased to avoid high mercury levels. In this way, mercury emissionreduction is achieved and excessive use of sorbent (with concomitantincrease of ash) is avoided.

Mercury is monitored in the convective pathway at suitable locations. Invarious embodiments, mercury released into the atmosphere is monitoredand measured on the clean side of the particulate control system.Mercury can also be monitored at a point in the convective pathwayupstream of the particulate control system. Experiments show that asmuch as 20 to 30% of the mercury in coal is captured in the ash and notreleased into the atmosphere when no mercury sorbent is added. Inpreferred embodiments, addition of mercury sorbents described hereinraises the amount of mercury capture to 90% or more. Mercury emissionsinto the atmosphere are correspondingly reduced.

In various embodiments, sorbent components or a sorbent composition isadded more or less continuously to the coal before combustion, to thefurnace during combustion, and/or to the convective pathway in the 1500°F. to 2700° F. zone as described above. In various embodiments,automatic feedback loops are provided between the mercury monitoringapparatus and the sorbent feed apparatus. This allows for a constantmonitoring of emitted mercury and adjustment of sorbent addition ratesto control the process.

In preferred embodiments, mercury and sulfur are monitored usingindustry standard methods such as those published by the AmericanSociety for Testing and Materials (ASTM) or international standardspublished by the International Standards Organization (ISO). Anapparatus comprising an analytical instrument is preferably disposed inthe convective pathway downstream of the addition points of the mercuryand sulfur sorbents. In a preferred embodiment, a mercury monitor isdisposed on the clean side of the particulate control system.Alternatively or in addition, the flue gases are sampled at appropriatelocations in the convective pathway without the need to install aninstrument or monitoring device. In various embodiments, a measuredlevel of mercury or sulfur is used to provide feedback signals to pumps,solenoids, sprayers, and other devices that are actuated or controlledto adjust the rate of addition of a sorbent composition into the coalburning system. Alternatively or in addition, the rate of sorbentaddition can be adjusted by a human operator based on the observedlevels of mercury and/or sulfur.

In various embodiments, the ash produced by burning coal in the presenceof the sorbents described herein is cementitious in that it sets anddevelops strength when combined with water. The ash tends to beself-setting due its relatively high level of calcium. The ash servesalone or in combination with portland cement as a hydraulic cementsuitable for formulation into a variety of cementitious mixtures such asmortars, concretes, and grouts.

The cementitious nature of ash produced as described herein isdemonstrated for example by consideration of the strength activity indexof the ash, or more exactly, of a cementitious mixture containing theash. As described in ASTM C311-05, measurement of the strength activityindex is made by comparing the cure behavior and property development ofa 100% portland cement concrete and a test concrete wherein 20% of theportland cement is replaced with an equal weight of a test cement. Inthe standard test, strength is compared at 7 days and at 28 days. A“pass” is considered to be when the strength of the test concrete is 75%of the strength of the portland cement concrete or greater. In variousembodiments, ashes of the invention exhibit of strength activity of 100%to 150% in the ASTM test, indicating a strong “pass”. Similar highvalues are observed when tests are run on test mixtures with other thanan 80:20 blend of portland cement to ash. In various embodiments, astrength activity index of 100% to 150% is achieved with blends of 85:15to 50:50, where the first number of the ratio is portland cement and thesecond number of the ratio is ash prepared according to the invention.In particular embodiments, the strength development of an all-ash testcementitious mixture (i.e., one where ash represents 100% of the cementin the test mixture) is greater than 50% that of the all-portland cementcontrol, and is preferably greater than 75%, and more preferably 100% ormore, for example 100-150%. Such results demonstrate the highlycementitious nature of ash produced by burning coal or other fuel in thepresence of the sorbent components described herein.

Because the ash resulting from combustion of coal according to theinvention contains mercury in a non-leaching form, it is available to besold into commerce. Non-limiting uses of spent or waste fly ash orbottom ash include as a component in a cement product such as portlandcement. In various embodiments, cement products contain from about 0.1%up to about 99% by weight of the coal ash produced by burningcompositions according to the invention. In one aspect, the non-leachingproperty of the mercury and other heavy metals in the coal ash makes itsuitable for all known industrial uses of coal ash.

Coal ash according to the invention, especially the fly ash collected bythe particle control systems (bag house, electrostatic precipitators,etc.) is used in portland cement concrete (PCC) as a partial or completereplacement for portland cement. In various embodiments, the ash is usedas a mineral admixture or as a component of blended cement. As anadmixture, the ash can be total or partial replacement for portlandcement and can be added directly into ready mix concrete at the batchplant. Alternatively, or in addition, the ash is inter-ground withcement clinker or blended with portland cement to produce blendedcements.

Class F and Class C fly ashes are defined for example in U.S. StandardASTM C 618. The ASTM Standard serves as a specification for fly ash whenit is used in partial substitution for portland cement. It is to benoted that coal ash produced by the methods described herein tends to behigher in calcium and lower in silica and alumina than called for in thespecifications for Class F and Class C fly ash in ASTM C 618. Typicalvalues for the fly ash of the invention is >50% by weight CaO, and <25%SiO₂/Al₂O₃/Fe₂O₃. In various embodiments, the ash is from 51 to 80% byweight CaO and from about 2 to about 25% of total silica, alumina, andiron oxide. It is observed that fly ash according to the invention ishighly cementitious, allowing for substitutions or cutting of theportland cement used in such cementitious materials and cementitiousmaterials by 50% or more. In various applications, the coal ashresulting from burning coal with sorbents described herein issufficiently cementitious to be a complete (100%) replacement forportland cement in such compositions.

To further illustrate, the American Concrete Institute (ACI) recommendsthat Class F fly ash replace from 15 to 25% of portland cement and ClassC fly ash replace from 20 to 35%. It has been found that coal ashproduced according to the methods described herein is sufficientlycementitious to replace up to 50% of the portland cement, whilemaintaining 28 day strength development equivalent to that developed ina product using 100% portland cement. That is, although in variousembodiments the ash does not qualify by chemical composition as Class Cor Class F ash according to ASTM C 618, it nevertheless is useful forformulating high strength concrete products.

Coal ash made according to the invention can also be used as a componentin the production of flowable fill, which is also called controlled lowstrength material or CLSM. CLSM is used as a self-leveling,self-compacting back fill material in place of compacted earth or otherfill. The ash described herein is used in various embodiments as a 100%replacement for portland cement in such CLSM materials. Suchcompositions are formulated with water, cement, and aggregate to providea desired flowability and development of ultimate strength. For example,the ultimate strength of flowable fill should not exceed 1035 kPa (150pounds per square inch) if removability of the set material is required.If formulated to achieve higher ultimate strength, jack hammers may berequired for removal. However, when it is desired to formulate flowablefill mixes to be used in higher load bearing applications, mixturescontaining a greater range of compressive strength upon cure can bedesigned.

Coal ash produced according to the methods described herein is alsousable as a component of stabilized base and sub base mixtures. Sincethe 1950's numerous variations of the basic lime/fly ash/aggregateformulations have been used as stabilized base mixtures. An example ofthe use of stabilized base is used as a stabilized road base. Toillustrate, gravel roads can be recycled in place of using ash accordingto the composition. An existing road surface is pulverized andre-deposited in its original location. Ash such as produced by themethods described herein is spread over the pulverized road material andmixed in. Following compaction, a seal coat surface is placed on theroadway. Ash according to the invention is useful in such applicationsbecause it contains no heavy metals that leach above regulatoryrequirements. Rather, the ash produced by methods of the inventioncontains less leachable mercury and less leachable other heavy metals(such as arsenic and lead) than does coal ash produced by burning coalwithout the sorbents described herein.

Thus, the invention provides various methods of eliminating the need tolandfill coal ash or fly ash resulting from combustion of coal thatcontains high levels of mercury. Instead of a costly disposal, thematerial can be sold or otherwise used as a raw material.

In a preferred embodiment, use of the sorbents results in a cementitiousash that can replace portland cement in whole or in part in a variety ofapplications. Because of the re-use of the cementitious product, atleast some portland cement manufacture is avoided, saving the energyrequired to make the cement, and avoiding the release of significantamounts of carbon dioxide which would have arisen from the cementmanufacture. Other savings in carbon dioxide emissions result from thereduced need for lime or calcium carbonate in desulfurization scrubbers.The invention thus provides, in various embodiments, methods for savingenergy and reducing green house emissions such as carbon dioxide.Further detail of various embodiments of this aspect of the inventionare given below.

Portland cement is manufactured in a wet or a dry process kiln. Whilethe wet and dry processes differ, both processes heat the raw materialin stages. Cement manufacturing raw materials comprise sources ofcalcium, silica, iron, and alumina, and usually include limestone, aswell as a variety of other materials, such as clay, sand, and/or shale,for example. The first stage is a pre-heating stage that drives off anymoisture from the raw materials, removes water of hydration, and raisesthe material temperature up to approximately 1500° F. The second stageis the calcination stage which generally occurs between about 1500° F.and 2000° F., where the limestone (CaCO₃) is converted to lime (CaO) bydriving off carbon dioxide (CO₂) in a calcination reaction. The rawmaterials are then heated to a maximum temperature of between about2500° F. to 3000° F. in the burning zone, where they substantially meltand flux, thus forming inorganic compounds, such as tricalcium silicate,dicalcium silicate, tricalcium aluminate, and tetracalciumaluminoferrite. A typical analysis of portland cement products showsthat they contain approximately 65-70% CaO, 20% SiO₂, 5% Al₂O₃, 4%Fe₂O₃, with lesser amounts of other compounds, such as oxides ofmagnesium, sulfur, potassium, sodium, and the like. The molten rawmaterial is cooled to solidify into an intermediate product in smalllumps, known as “clinker” that is subsequently removed from the kiln.Clinker is then finely ground and mixed with other additives (such as aset-retardant, gypsum) to form portland cement. Portland cement can thenbe mixed with aggregates and water to form concrete.

Cement production is an energy sensitive process in which a combinationof raw materials is chemically altered through intense heat to form acompound of binding properties. Cement manufacturing is the largestnon-energy industrial source of carbon dioxide emissions. The emissionsresult from heating limestone, which constitutes approximately 80% ofthe feed to cement kilns. During cement production, high temperaturesare used to transform the limestone into lime, releasing carbon dioxideinto the atmosphere. In this process, one molecule of calcium carbonateis decomposed into one molecule of carbon dioxide gas and one moleculeof calcium oxide.

The cement manufacturer utilizes nearly 100% of the calcium oxideobtained from calcinated calcium carbonate. Thus, the amount of calciumoxide in the cement clinker is a good measure of the carbon dioxideproduced during production. In an example, to estimate carbon dioxideemission from cement production, an emission factor is derived bymultiplying the fraction of lime in the cement clinker by a constantthat reflects the mass of carbon released per unit of lime. In oneexample, assuming an average lime content of 64.6% based onrecommendations of the International Panel for Climate Control, anemissions factor of 0.138 tons of carbon per ton of clinker produced isobtained. Additional carbon dioxide may be released as a result ofadding extra lime to make masonry cement; a more plastic cement thattypically is used in mortar.

In cement making, carbon dioxide emissions result from energy use andfrom decomposition of calcium carbonate during clinker production.Depending on the fuel source that provides the energy, carbon dioxideemissions may vary. For example, the use of a cleaner burning fuel, suchas natural gas, produces less carbon dioxide emissions than the use of afuel such as coal. In various embodiments, the invention described abovemay be used in the production of cement. In such embodiments, the use ofthe invention in the production of cement will reduce carbon dioxideemissions.

In various embodiments, the invention described herein, may be used inthe production of cement to produce carbon dioxide emission credits bylowering carbon dioxide emission in the production of cement. Inpreferred embodiments, a point source for air emissions, such as acement plant or a coal-fired power plant, is brought into compliancewith the Kyoto protocol.

Although the invention is not to be limited by theory, it is believedthat the sorbent compositions described above provide additional orsupplemental sources of silica and alumina into the coal burningprocess. Combustion of the coal with the added silica and alumina formsa geopolymeric matrix such as is known in cold ceramics. Although coalnaturally contains small amounts of silica and/or alumina, it isbelieved that the amount of the materials naturally occurring in coal isnormally not sufficient to provide the geopolymeric matrix uponcombustion. Further, the silica and alumina naturally occurring in coalis not necessarily balanced with the natural occurring calcium in orderto provide optimum sulfur and/or mercury capture and/or cementitious ashproduct upon combustion.

In various embodiments, the invention provides methods for improving theleaching quality of heavy metals such as mercury from coal. The methodsinvolve adding sufficient silica and/or alumina to the coal to cause ageopolymer to form upon combustion. Preferably, the silica and aluminaare added along with sufficient alkali powders to reduce sulfur pitting.The alkali powders tend to neutralize the silica and alumina, withformation of geopolymeric ash along with coupling silica and/or aluminato form a ceramic like matrix that reports as a stabilized ash. It mayalso be that the alumina and silica burning with the coal forms arefractory like mixture compound with mercury, lead, arsenic, cadmium,antimony, cobalt, copper, manganese, zinc, and/or other heavy metals. Asa result, the resulting coal ash or fly ash containing heavy metals isresistant to leaching under standard conditions. As noted above, thenon-leaching quality of the coal ash leads to commercial advantagesbecause the product would no longer be considered as a hazardousmaterial.

EXAMPLES

In Examples 1-6, coals of varying BTU value, sulfur, and mercury contentare burned in the CTF furnace at the Energy Environmental ResearchCenter (EERC) at the University of North Dakota. Percent mercury isreported based on the total amount of the element in the coal prior tocombustion. Percentage sulfur removal is a percentage reduction abovebaseline, the baseline determined by measuring sulfur emissions fromburning without sorbent.

Example 1

This example illustrates the mercury sorption ability of a calciumbromide/water solution when applied to a Powder River basinsub-bituminous coal. The as-fired coal has a moisture content of 2.408%,ash content of 4.83%, sulfur content of 0.29%, a heating value of 8,999BTU and a mercury content of 0.122 μg/g. Combustion without sorbentresults in a mercury concentration of 13.9 μg/m³ in the exhaust gas. Thefuel is ground to 70% passing 200 mesh and blended with 6% of a sorbentpowder and 0.5% of a sorbent liquid, based on the weight of the coal.The powder contains by weight 40-45% portland cement, 40-45% calciumoxide, and the remainder calcium- or sodium montmorillonite. The liquidis a 50% by weight solution of calcium bromide in water.

The sorbents are mixed directly with the fuel for three minutes and thenstored for combustion. The treated coal is fed to the furnace.Combustion results in a 90% mercury (total) removal at the bag houseoutlet and a 80% removal of sulfur as measured at the bag house outlet.

Example 2

This example illustrates the use of powder and liquid sorbents appliedto three bituminous coals of varying mercury content. All coals areprepared as in example #1, with the same addition levels of sorbents.

% of Mercury % Sulfur Parameter Coal Removal Removal % Moisture 8.48Pittsburgh, Seam, 97.97 40.0 % Sulfur 2.28 Bailey Coal Mercury 16.2 μg/m³ BTU value 13,324 % Moisture 10.46 Freeman Crown 97.9 36.0 % Sulfur4.24 III Mercury 8.53 μg /m³ BTU value 11,824 % Moisture 1.0 KentuckyBlend 90.1 52.0 % Sulfur 1.25 Mercury 5.26 μg /m³ BTU value 12,937

Example 3

This example illustrates addition of a mercury sorbent post-combustion.Pittsburgh Seam-Bailey Coal is ground to 70% passing 200 mesh. Nosorbent was added to the fuel pre-combustion. Liquid sorbent containing50% calcium bromide in water is duct injected into the gaseous stream ofthe furnace in the 2200° F.-1500° F. zone. The liquid sorbent isinjected at the rate of approximately 1.5% by weight of the coal.

Sorbent % S # Hg Coal Type Composition reduction Reduction PittsburghSeam- 50% CaBr₂ 28.13 96.0 Bailey Coal 50% H₂O

Example 4

This example illustrates addition of a liquid and a powder sorbentpost-combustion. No sorbent was added directly to the fuel. Both fuelsare bituminous and noted as Freeman Crown III and Pittsburgh Seam-BaileyCoal. In both cases the coal was ground to 70% minus 200 mesh prior tocombustion. The powder and liquid sorbents are as used in Example 1.Rates of liquid and powder addition (percentages based on the weight ofthe coal being burned), as well as mercury and sulfur reduction levels,are presented in the table.

Liquid Powder sorbent sorbent injection injection S Hg Coal Type raterate Reduction Reduction Freeman Crown III 1.0 4.0 36.27 97.89Pittsburgh Seam- 1.5 6.10 33.90 96.00 Bailey Coal

Example 5

Pittsburgh Seam Bailey Coal is prepared as in Example 1. The powdersorbent of Example 1 is added to the coal pre-combustion at 9.5% byweight. The liquid sorbent of Example 1 (50% calcium bromide in water)is injected post-combustion in the 1500° F.-2200° F. zone at a rate of0.77%, based on the burn rate of the coal. Sulfur reduction is 56.89%and mercury reduction is 93.67%.

Example 6

Kentucky Blend Coal is prepared as in Example 1. The powder sorbent ofExample 1 is added to the coal pre-combustion at 6% by weight. Theliquid sorbent of Example 1 (50% calcium bromide in water) is injectedpost-combustion in the 1500° F.-2200° F. zone at a rate of 2.63%, basedon the burn rate of the coal. Sulfur reduction is 54.91% and mercuryreduction is 93.0%.

In Examples 7-10, coals of varying BTU value, sulfur, and mercurycontent are burned in a variety of boilers at electrical utilities.Percent mercury reduction is reported based on the total amount of theelement in the coal prior to combustion. Percentage sulfur removal is apercentage reduction above baseline, the baseline determined bymeasuring sulfur emissions from burning without sorbent.

Example 7

A fuel containing bituminous and sub-bituminous coals, pet coke, woodchips, and rubber tire scraps is burned in a stoker furnace to produce60 megawatts of power. The furnace is operating in a balanced draftmanner. Baseline emissions of mercury from burning the fuel with noadded sorbent indicate the mercury is mostly in the oxidized form. Afterestablishing a baseline, a powder sorbent composition is added at atreat rate of 5.5-6% by weight to the furnace about two feet above thegrate through the fly ash recycle/reinjection tubes of the furnace.After steady state is reached, the mercury capture is 96%. The powdersorbent composition is 93% by weight of a 50:50 mixture of cement kilndust and lime kiln dust; and 7% by weight calcium montmorillonite. Whilemaintaining addition of the powder sorbent composition, a liquid sorbentcontaining 50% by weight calcium bromide in water is added onto the fuelprior to combustion at a rate of 0.5%, based on the weight of the fuelbeing consumed. On addition of the liquid sorbent, the mercury captureincreases to 99.5%.

Example 8

PRB coal (pulverized to 75% passing 200 Mesh) is burned in atangentially fired boiler operating in a balanced draft manner toproduce 160 megawatts of electric power. After establishing baseline Sand Hg emissions from burning of the coal without added sorbent, apowder sorbent composition as in Example 7 is added at a treat level of5.5-6% into the furnace. Addition is made through a lance positioned 4feet from the inside wall of the furnace and 20 feet above the fireball.The temperature of the flue gases at the point of injection is about2400° F. to 2600° F., measured by a temperature sensor. Sulfur captureis increased by 65% over baseline. Mercury capture is 3%, based on thetotal amount of mercury in the PRB coal. Then, while continuing additionof the powder sorbent, a 50% calcium bromide in water solution is addedto the pulverized coal in the coal feeders by drip feed at a treat rateof 0.5%, based on the rate of coal consumption. Mercury captureincreases to 90%.

Example 9

PRB coal pulverized to about 200 mesh is burned in a tangentially firedfurnace operating in a positive draft manner to produce about 164megawatts of electric power. After establishing baseline sulfur andmercury emissions from burning the coal without added sorbent (themercury is predominantly in elemental form in the flue gas emissions), apowder sorbent composition as in Example 7 is added at a treatment rateof 5.5-6.0% by weight of fuel into the furnace just below the neck ofthe furnace, about 20 feet above the fireball. The temperature at theinjection point is about 3000° F. to 3300° F. Addition is made through aseries of 3 lances along one side of the furnace. Each lance conveysapproximately the same amount of powder and protrudes approximately 3feet into the furnace from the inside wall. Sulfur capture is increasedby 50% over baseline. Mercury capture is approximately 1-3% overbaseline. While continuing addition of the powder sorbent compositioninto the furnace, a 50% by weight solution of calcium bromide in wateris added directly to the fuel in the fuel feeder at a rate ofapproximately 0.2% by weight, based on the weight of coal being consumedby combustion. Mercury capture increases to 90%.

Example 10

The same process as Example 9 is also followed except the powder sorbentis added directly to the coal feeders (upstream of the furnace) ratherthan directly into the furnace. The same sulfur and mercury reductionsare observed as with Example 9.

Example 11

PRB coal is burned in a positive draft tangentially fired boiler togenerate electricity for consumer use. Powdered coal (75% passing 200mesh) is fed to the boiler. Before introduction of the powdered coalinto the boiler, a powder sorbent is added to the coal at a rate of 6%by weight, based on the rate of coal being consumed by combustion. Thepowder sorbent contains 93% by weight of a 50/50 mixture of cement kilndust and lime kiln dust, and 7% by weight of calcium montmorillonite. Atthe same time, a 50% by weight solution of calcium bromide in water isdripped onto the coal at a rate of 0.1 to 2% by weight based on the rateof consumption of coal by combustion. Fly ash samples are collectedbefore addition of sorbents (baseline), and after addition of the powderand liquid sorbents. The levels of chlorine and heavy metals aredetermined according to standard methods. Results are in the table(Table 1).

TABLE 1 Fly Ash Composition with and without sorbents Test - Aftersorbent addition Baseline - Prior to (ppm except for sorbent additionElement chlorine) (ppm except for chlorine) As 59.3 26.5 Ba 1.3 1.4 Cd2.3 1.1 Co 44.8 38.5 Cr 52.0 34.3 Cu 61.0 48.8 Mn 455.7 395.5 Mo 26.031.5 Ni 208.5 325.5 Pb 45.8 31.3 Sb 23.0 7.3 V 473.0 874.5 Zn 3954.0974.7 Mercury 0.246 0.128 Chlorine 0.940% 0.56%

It is seen that use of the sorbents increases the level of several heavymetals found in the fly ash. For example, arsenic, cadmium, chromium,lead, mercury, and chlorine are present at higher levels in the test ashthan in the baseline. This is believed to represent increased capture ofthe elements in the ash. The increased level of zinc in the test ash isunexplained. However, it could be due to the fact that a great deal ofde-slagging is observed from the boiler tubes upon use of sorbents ofthe invention. It could be that the increased levels of zinc areattributable to material removed from the boiler tubes during combustionwith the sorbents.

Example 12

Next the ash samples are tested according to the TCLP procedure of theU.S. Environmental Protection Agency (EPA) to determine the acidleaching thresholds of key elements. Results are in Table 2.

TABLE 2 Fly Ash TCLP Test Results Baseline - prior EPA to sorbent Test -with Threshold Limit addition sorbent addition Element (ppm) (ppm) (ppm)Arsenic 5.0 <0.04 <0.04 Barium 100.0 0.814 0.313 Cadmium 1.0 <0.04 <0.04Chromium 5.0 0.030 <0.007 Lead 5.0 0.513 0.096 Mercury 0.20 0.095 0.078Selenium 1.0 <0.07 <0.07 Silver 5.0 3.835 3.291

Table 2 shows that, while the ash is higher in absolute levels ofelements such as arsenic, lead, and mercury, nevertheless the amount ofleachable arsenic, lead, and mercury is actually lower in the test ashthan in the baseline.

Example 13

PRB coal (75% passing 200 mesh) is burned in a balanced drafttangentially fired furnace to produce 160 MWatts of power. The coal isburned for a time period to produce 8 box cars of fly ash. A powdersorbent is added at a rate of 4-6% by weight into the system during thetime period. For the first third of the time period, addition of sorbentis solely into the furnace at a location just below the nose of thefurnace through a lance inserted through the furnace wall; for the nextthird, addition of sorbent is half into the furnace and half onto thepowdered coal pre-combustion; for the final third, addition of powdersorbent is 100% onto the coal pre-combustion. Throughout the timeperiod, a liquid sorbent (50% by weight calcium bromide in water) isadded onto the powdered coal pre-combustion at a rate of 0.15% byweight, based on the rate of coal consumption. A consolidated samplerepresentative of the eight box cars of ash is collected and measuredfor leaching using the US EPA TCLP procedure. The leaching result forbarium is 26 ppm, well below the regulatory level of 100 ppm. TCLPvalues for As, Cd, Cr, Pb, Hg, Se, and Ag are below the detection limitsof the test. In particular, mercury leaching is <0.0020 ppm, which isless than 2 ppb.

What is claimed is:
 1. A method of improving the leaching quality of ashproduced by burning mercury-containing coal, comprising adding silicaand alumina to the coal in a sufficient amount to cause a geopolymer toform upon combustion, combusting the coal in the presence of the addedsilica and alumina to produce coal ash and heat energy, and measuringthe leaching of mercury from the ash.
 2. A method according to claim 1,wherein the coal ash is fly ash.
 3. A method according to claim 1,comprising adding a sorbent comprising silica and alumina onto the coalupstream of the furnace.
 4. A method according to claim 3, wherein thesorbent comprises alkali powders.
 5. A method according to claim 1,wherein the ash sequesters one or more of mercury, lead, arsenic,cadmium, antimony, cobalt, copper, manganese, and zinc.
 6. A methodaccording to claim 1, comprising adjusting the amount of silica andalumina added depending on the measured level of leaching.
 7. A methodaccording to claim 3, wherein the sorbent comprises greater than 5 wt. %alumina.
 8. A method according to claim 3, wherein the sorbent comprisesgreater than 8 wt. % alumina.
 9. A method according to claim 1, furthercomprising adding a halogen compound onto the coal upstream of thefurnace.
 10. A method according to claim 9, wherein the halogen compoundcomprises calcium bromide.
 11. A method according to claim 1, comprisingadding an aluminosilicate material selected from micas, clays, talc, andchrysotiles onto the coal.
 12. A method according to claim 11,comprising adding a clay selected from chlorites, glauconite, illite,polygorskite, pyrophillite, sauconite, vermiculite, kaolinite, calciummontmorillonite, and sodium montmorillonite.
 13. A method of increasingthe cementitious properties of coal ash while reducing mercury emissionsarising from the combustion of coal, comprising adding a sorbentcomposition onto the coal prior to combustion and/or adding a sorbentcomposition into the furnace along with the coal, wherein the sorbentcomposition comprises calcium, silica, alumina, iron oxide, and alkalisselected from Na₂O and K₂O.
 14. A method according to claim 13, whereinthe sorbent comprises 2-15 wt. % Al₂O₃, 30-75 wt. % CaO, 5-20 wt. %SiO₂, 1-10 wt. % Fe₂O₃, and 0.1-5 wt. % alkalis.
 15. A method accordingto claim 14, wherein the sorbent comprises 2-10 wt. % Al₂O₃, 40-70 wt. %CaO, 5-15 wt. % SiO₂, and 2-9 wt. % Fe₂O₃.
 16. A method according toclaim 13, wherein the sorbent composition comprises cement kiln dust.17. A method according to claim 13, further comprising adding a halogencompound onto the coal upstream of the furnace.
 18. A method accordingto claim 17, wherein the halogen compound comprises a bromine compound.19. A method according to claim 18, wherein the halogen compoundcomprises calcium bromide.
 20. A method according to claim 13, furthercomprising recycling the coal ash for use in portland cementmanufacture.